Ligands for the alkali metals. Part 4. Nuclear magnetic resonance of crown ethers with alkali-metal ions

Abstract

The solution ¹H n.m.r. spectra of the crown ligands 2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclo-pentadecin(benzo-15-crown-5) and 2,3,5,6,8,9,11,12,14,15,17,18-dodecahydro-1,4,7,10,13,16,19-benzoheptaoxacycloheneicosin (benzo-21-crown-7) as free ligands and in mixtures with alkali-metal iodides or thiocyanates are reported and analysed to provide shifts and coupling constants. The signals of some or all the hydrogens in the mixtures are upfield of those of the free ligands for complexes of 2 : 1 ligand: cation ratio, but downfield when only a 1 : 1 complex is present, and this finding may be used diagnostically. An empirical explanation of the direction of the shift on complexation is given in terms of the electric-field effect, ring-current variations, and specific ion pairing. The gross conformation of the two complexes in solution is shown to be similar to that found in the crystal by X-ray methods.